1. Field of the Invention
This invention relates to a method of forming a silicon-based semiconductor layer, and a photovoltaic element produced by using the method. More particularly, this invention relates to a photovoltaic element such as a solar cell or a sensor, in which at least one set of a pin junction (or nip junction) has been deposited and which has high conversion efficiency and may cause less degradation even for long-time outdoor use, and also relates to a production method therefor, a construction material and a power generation system.
2. Related Background Art
As individual power sources for electric instruments or as substitute energy sources of system electric power, various photovoltaic elements have already been put into use. However, especially as a substitute for system electric power, photovoltaic elements are still in a high cost per power generation quantity. Accordingly, research and development concerning photovoltaic elements are energetically made in succession also at present.
For example, with regard to materials used at the most important part that takes over photoelectric conversion, technical research and development are being made on crystal type photovoltaic elements and thin-film type photovoltaic elements. The crystal type photovoltaic elements make use of single-crystal or polycrystalline silicon, and the thin-film type photovoltaic elements make use of amorphous or microcrystalline silicon, silicon-germanium or silicon carbide or compound semiconductors. Research and development have been made forward on the microcrystalline silicon, which, however, has behind been put into practical use compared with crystal materials and non-crystal materials.
In recent years, however, the microcrystalline silicon attracts notice in that good photoelectric conversion efficiency can be attained and any photodegradation is not seen at all, as reported by J. Meier, P. Torres, R. Platz, H. Keppner, A. Shah et al. (Mat. Res. Soc. Symp. Proc. Vol. 420, p. 3, 1996; hereinafter, referred to as “Document 1”).
According to this Document 1, a photovoltaic element is produced by plasma-assisted chemical vapor deposition (plasma CVD) in which high-frequency power having a frequency of 70 MHz is supplied. This photovoltaic element is so constructed as to have one set of semiconductor junction. Then, it is reported that, in this photovoltaic element, a photoelectric conversion efficiency of 7.7% is attained and any photodegradation is not seen at all. The Document 1 further discloses that an initial photoelectric conversion efficiency of 13.1% and a photodegradation rate of −12.4% have been attained where a stacked photovoltaic element is produced in which amorphous silicon and microcrystalline silicon are placed one upon another.
In addition to the foregoing, there is a photovoltaic element reported by K. Yamamoto, A. Nakashima, T. Suzuki, M. Yoshimi, H. Nishio and M. Izumina (Jpn. J. Appl. Phys. Vol. 33, 1994, pp. L1751-L1754, Part 2, No. 12B, 15 Dec. 1994; hereinafter, referred to as “Document 2”). This report shows an example in which a p-type layer doped with boron in a high concentration is subjected to laser annealing and a columnar microcrystalline structure is provided thereon.
As a method of forming a silicon semiconductor layer which shows crystallizability, a method such as casting has conventionally been carried out in which the layer is grown from a liquid phase. This method, however, requires high-temperature treatment, and has had a subject for mass production and cost reduction.
As a method of forming a silicon semiconductor layer which shows crystallizability, other than the casting, Japanese Patent Application Laid-Open No. 5-136062 (hereinafter, referred to as “Document 3”) discloses a method in which amorphous silicon having been formed is subjected to hydrogen plasma processing and this is repeated to form a polycrystalline silicon film.
In general, in photovoltaic elements making use of the silicon semiconductor layer which shows crystallizability, it is known that the mobility of carriers is impaired because of, e.g., the influence of dangling bonds and so forth of silicon at crystal grain boundaries, any strain or distortion produced in the vicinity of grain boundaries, and any imperfectness of crystals themselves to adversely affect photoelectric characteristics required as photovoltaic elements.
As a countermeasure for bringing the photoelectric characteristics to be less affected, it is considered effective to improve crystallinity and crystallizability and to make crystal size larger to lower the density of crystal grain boundaries. As a means for the achievement of these, it must be designed to lower film-forming rate or to form films while repeating the formation of silicon film and its annealing in an atmosphere of hydrogen.
Japanese Patent Application Laid-Open No. 3-177077 (hereinafter, referred to as “Document 4”) also discloses a technique in which an i-type semiconductor layer comprised of amorphous silicon is formed in a stacked construction consisting of two different semiconductor layers (a) and (b). Here, one semiconductor layer (a) is formed as a layer mainly composed of silicon atoms and containing hydrogen atoms in an amount of from 0.1 to 10 atomic % and fluorine atoms of from 0.01 to 0.5 atomic %, and the other semiconductor layer (b) is formed as a layer mainly composed of silicon atoms and containing hydrogen atoms in an amount of from 20 to 40 atomic % and formed in a layer thickness of from 10 to 100 A. Thus, any film surface defects can be compensated, and a photoelectric conversion device having superior characteristics can be obtained.
This technique has made it possible to greatly improve the characteristics of amorphous silicon film.
However, the techniques disclosed in Documents 1 to 4 have the following problems. That is, in the case of the microcrystalline silicon (μC—Si) photovoltaic element of Document 1, although any photodegradation is not seen at all, only low conversion efficiency, i.e., a short-circuit current of 25.4 mA/cm2 and a photoelectric conversion efficiency of 7.7% have been achieved for a layer thickness of 3,600 nm. In addition, the μC—Si layer has a layer thickness of as large as 3,600 nm. Besides, its deposition rate is as low as 0.12 nm/sec, and it must take about 8 hours to form the layer. Thus, there is a problem that this photovoltaic element is not on an industrially practical level.
In the case of the technique of Document 2, only much lower conversion efficiency, i.e., a short-circuit current of 14.3 mA/cm2 and a photoelectric conversion efficiency of 7.7% have been achieved in a layer thickness of 2,000 nm.
The technique disclosed in Document 3 also provides the cause of a longer film formation time and a higher cost.
In addition, the technique disclosed in Document 4 can not necessarily be a sufficient measure to solve the problem on photodegradation because the semiconductor layer is formed of amorphous silicon, thus there is room for further improvement.
Now, in recent years, the group who has reported in Document 2 has advanced the technique, and reports that a short-circuit current of 26.12 mA/cm2 and a photoelectric conversion efficiency of 9.8% have been achieved in a layer thickness of 3,500 nm (14th European Photovoltaic Solar Energy Conference, Barcelona, Spain, Jun. 30 to 4 Jul. 1997, pp. 1018-1021). However, the conversion efficiency and the productivity are both still insufficient.